A delay in a mobile network is a key performance indicator (KPI) of a network and directly affects user experience. Continuously emerging new services (for example, a service of the Internet of Vehicles) also impose an increasingly high requirement for the delay.
For example, some end-to-end services have the following requirements for the delay:
It is required that an event trigger delay in an interactive sports game is less than 25 ms;
it is required that a delay of communication between self-driving cars is less than 30 ms;
it is required that a round trip time (RTT) delay in remote control is less than 50 ms;
it is required that a delay in smart grid power automation protection is less than 8 ms; and
it is required that a call setup delay in public safety is less than 300 ms, and an end-to-end (E2E) media file transmission delay is less than 150 ms.
Efforts are continuously made to reduce the delay in an evolution process of a mobile communications standard. In an air interface technology, a scheduling interval at a physical layer affects the delay most obviously. The scheduling interval is 10 ms in Wideband Code Division Multiple Access (WCDMA), is shortened to 2 ms in High Speed Packet Access (HSPA), and is shortened to 1 ms in Long Term Evolution (LTE). Because of a requirement for a low-delay service, a short transmission time interval (TTI) frame structure needs to be introduced into an LTE physical layer. To further shorten the scheduling interval, a TTI may be shortened from 1 ms to one symbol to 0.5 ms. The symbol mentioned above may be an orthogonal frequency division multiplexing (OFDM) symbol in an LTE system.
As shown in FIG. 1, FIG. 1 is a delay diagram of a round trip time (RTT) for transmitting data on one symbol. A data transmission RTT is eight symbols. Based on a hybrid automatic repeat request (HARQ) technology, if a base station transmits data to user equipment on a symbol 3, and the user equipment correctly performs demodulation and decoding on received data, the user equipment feeds back an acknowledgement (ACK) character to the base station on a symbol 7; or if the user equipment does not correctly perform demodulation and decoding on received data, the user equipment feeds back a negative acknowledgement (NACK) character to the base station on a symbol 7, and the base station confirms, on a symbol 11, that the ACK/NACK is received. The ACK/NACK is referred to as HARQ feedback information. There are three possible cases in which the user equipment sends the HARQ feedback information. Case 1: The user equipment does not receive, on the symbol 3, a DL grant of a data packet scheduled by the base station, and therefore does not send the HARQ feedback information on the symbol 7. Case 2: The user equipment receives, on the symbol 3, a data packet scheduled by the base station, but does not correctly perform decoding, and therefore send the NACK on the symbol 7. Case 3: The user equipment receives, on the symbol 3, a data packet scheduled by the base station, and correctly performs decoding, and therefore send the ACK on the symbol 7. Correspondingly, performance indicators of the HARQ feedback information include a false alarm and an erroneous detection. The false alarm means that the user equipment does not send the HARQ feedback information or sends the NACK, but the base station considers, by means of detection, that the user equipment sends the ACK, and this is corresponding to the foregoing Case 1 and Case 2. The erroneous detection means that the user equipment sends the ACK, but the base station does not detect the ACK, and this is corresponding to the foregoing Case 3. Usually, when performance of the HARQ feedback information is evaluated, performance of the erroneous detection is considered when a false alarm indicator is reached.
The base station may perform erroneous detection in a HARQ feedback information transmission process, thereby further affecting network performance. As the short TTI frame structure is introduced, there is a higher probability that the base station performs erroneous detection during data transmission in a short TTI than that during data transmission in a normal TTI of 1 ms. Therefore, the data transmission in the short TTI imposes greater impact on the network performance.
That the short TTI is one symbol is used as an example. Analysis is separately performed on impact on the network performance during ACK transmission in a TTI of one symbol and impact on the network performance during ACK transmission in a TTI of 1 ms. It is learned that when the ACK is transmitted by using the TTI of one symbol, when there is a false alarm, when a probability that the user equipment sends the ACK and the base station does not detect the ACK is 0.01, a signal-to-noise ratio of the user equipment is 6.3 dB; however, when the ACK is transmitted by using the TTI of 1 ms, when there is a same false alarm, when a probability that the user equipment sends the ACK and the base station does not detect the ACK is also 0.01, a signal-to-noise ratio of the user equipment is −7.5 dB. By comparison, performance of the ACK when the ACK is transmitted by using the TTI of one symbol is approximately 14 dB less than performance of the ACK when the ACK is transmitted by using the TTI of 1 ms.
Therefore, when data transmission is performed in a short TTI, how to avoid a relatively high loss caused by the HARQ feedback information to the network performance is an urgent problem that needs to be resolved currently.